The present invention relates to systems for controlling the stability of a vehicle, commonly known as ESP(Electronic Stability Program) systems.
In safety systems for vehicles, it is necessary to be able to assess the handling of the vehicle in real time. This is the basis of the so-called ESP systems for controlling stability. These systems currently rely on, inter alia, monitoring movements of the vehicle by installing sensors in order to measure the lateral acceleration of the vehicle and the yaw velocity of the vehicle.
When moving under good safety conditions, that is to say when the stability of the vehicle is not compromised, the vehicle obeys the driver""s commands. If the driver, essentially as a result of his actions of steering the wheel, drives the vehicle beyond the limits of safety, the vehicle will exhibit oversteering or understeering. The vehicle turns, that is to say performs a yaw movement, in excess of that desired by the driver (oversteering) or less than desired by the driver (understeering).
Using a mathematical model of the tire and a mathematical model of the vehicle, and based on measurements supplied by sensors recording the actions of the driver of the vehicle (steering wheel angle, brake pedal depress, accelerator pedal depress) by speed sensors for the wheels, and on measurements of the lateral acceleration and yaw velocity, an ESP system constantly calculates the forces at the center of the wheels and estimates the grip potential of the road surface as a function of the lateral acceleration. Furthermore, the ESP system evaluates the handling of the vehicle, compares it to the handling desired by the driver, and corrects this handling if it establishes that the vehicle is not moving along a stable trajectory.
However, the use of tire models can introduce a certain number of approximations into the overall model. Furthermore, the fact that a control system is based on the displacements of the vehicle necessarily leads to a response a posteriori, which can be effective only after a delay depending on the inertia of the vehicle. It can be seen from this that an ESP system, since its variables include, inter alia, measurements of the lateral acceleration and the yaw velocity of the vehicle, first of all has to measure the displacement of the vehicle before deciding whether the displacement is within the bounds of stability or not, and can only then act on the operating means of the vehicle.
The system will detect a displacement of the vehicle not in accordance with the command given by the driver, the more slowly the greater the inertia of the vehicle, and the necessary correction will be all the more difficult the greater the inertia. At the present time the operating means are basically the vehicle""s brakes, controlled in this case wheel by wheel and outside the voluntary action of the driver, and the engine force, which can be reduced automatically by regulating the engine.
Furthermore, the detection of yaw movements requires the use of costly sensors. Also, existing systems have to estimate the grip of the wheels on the road surface in order to select the actuating parameters. This estimation deviates to a greater or lesser degree to the actual conditions.
The object of the present invention is to obviate the aforementioned disadvantages and, more particularly, to exclude completely the inertia of a vehicle in order to be able to act on the appropriate operating means so as to maintain the vehicle in a stable trajectory in accordance with the driver""s commands, by regulating the operating means in such a way that the actual forces acting at the center of each wheel correspond to the desired forces.
The invention provides a method for controlling the stability of a vehicle that has the advantage that it can be carried out without having to measure the yaw angle of the vehicle. The invention relates to a vehicle comprising a body and at least one front axle and one rear axle. A preferred but not limiting field of application of the invention is the case where each axle involves at least two suspension devices, each comprising one wheel, the said suspension devices being mounted on both sides of the mid-plane of symmetry of the vehicle, for example in the case of 4-wheeled touring vehicles. Each suspension device comprises a wheel, generally equipped with a tire or, which is the same in the context of the present invention, a non-pneumatic outer casing in contact with the ground. The vehicle is provided with operating means to act on the forces transmitted to the ground by each of the wheels, such as brakes, means for steering the wheels, optionally operating in a selective manner wheel by wheel, and distribution of the loads carried by each of the wheels.
According to a first embodiment of the invention, the method comprises the following steps:
measuring in real time the lateral forces xe2x80x9cYxe2x80x9d acting at the center of each of the front and rear wheels;
calculating for each of the wheels the desired lateral forces xe2x80x9cYdesiredxe2x80x9d on the basis of commands from the driver of the vehicle;
comparing the desired lateral forces with the measured lateral forces in order to obtain an error signal with respect to the desired lateral forces, and
if the forces acting on one of the axles do not correspond to the desired lateral forces, acting on the operating means so as to minimize the error signal.
The commands of the driver of the vehicle are intended to maintain the vehicle on a straight line trajectory regardless of the ambient disturbances (for example sidewind gusts), or are intended to cause the vehicle to execute a lateral displacement (change of lane for overtaking on a motorway) or to turn. Regardless of the operating means of the vehicle actuated by the driver (conventional steering wheel, operating lever as illustrated for example in patent application EP 0 832 807), the driver""s aim in fact is to impose certain lateral forces or certain variations of lateral forces. The invention accordingly involves measuring in real time the effective forces, comparing them with commands by the driver translated into lateral forces or variations in lateral forces, and as a result controlling appropriate operating means available on the vehicle.
According to a second embodiment of the invention, involving processing the driver""s commands differently (the reasons for which will be discussed in more detail hereinafter), the method comprises the following steps:
measuring in real time the lateral forces xe2x80x9cYxe2x80x9d acting at the center of each of the front and rear wheels, and calculating in real time the effective yaw moment exerted by the wheels on the vehicle;
measuring in real time a signal at the device for controlling the steering and calculating the desired yaw moment xe2x80x9cMdesiredxe2x80x9d;
comparing the effective and desired yaw moments in order to obtain an error signal with respect to the desired yaw moment; and
if the effective yaw moment is greater than the desired yaw moment, acting on the operating means so as to minimize the error signal.
Accordingly, if the lateral force of the front axle saturates, the vehicle will understeer since the lateral force (forces) of the front axle are less than the forces desired by the driver. An automatic action, for example of the type already known per se in conventional ESP systems (other types of actions will be discussed hereinafter) enables a resultant force to be exerted on the vehicle chassis in accordance with the driver""s wishes and thus enables understeering to be avoided.
If on the other hand it is the lateral force of the rear axle that saturates, then the vehicle will oversteer since the lateral forces of the rear axle are less than the forces desired by the driver. The said automatic action enables a resultant force to be exerted on the vehicle chassis in accordance with the driver""s wishes and thus enables oversteering to be avoided.
The above description relates to what is conventionally called a steady state (or established state). When considering a typical transient state involved in an emergency maneuver (avoiding an obstacle, changing lane), the steering wheel velocity may be regarded as equivalent to a desired yaw moment acting on the vehicle. If the actual yaw moment is less than the desired yaw moment, the vehicle will not turn sufficiently. If on the other hand the actual yaw moment is greater than the desired yaw moment, the vehicle will turn too much.
According to yet another embodiment of the invention, involving processing the driver""s commands differently so as to try to simulate the subjective perception of a driver, the method comprises the following steps:
measuring in real time the lateral forces xe2x80x9cYxe2x80x9d acting at the center of each of the front and rear wheels,
measuring in real time the angle at the steering wheel,
calculating in real time the yaw acceleration from the lateral forces xe2x80x9cYxe2x80x9d and the distances of the center of gravity of the vehicle from the front and rear wheels,
calculating in real time the gain in the yaw velocity with respect to the steering wheel velocity,
if the gain in the yaw velocity is less than a first low threshold, controlling the operating means in order to increase the steering of the vehicle, and if the gain in the yaw velocity is greater than a first high threshold, controlling the operating means in order to reduce the steering of the vehicle.
The expression xe2x80x9cthe gain in the yaw velocityxe2x80x9d is understood to mean the ratio of the variation of the yaw velocity to the variation of the angle at the steering wheel. It should be noted that the speed according to which the driver acts on the steering wheel corresponds to a need for a yaw acceleration of the vehicle. If the gain in the yaw velocity is less than a first low threshold, it is considered that the vehicle will begin to understeer dangerously. The object of the correction is to assist the steering. If the gain in the yaw velocity is greater than a first high threshold, it is considered that the vehicle will begin to oversteer dangerously. The object of the correction is to prevent excessive steering. These thresholds may be determined experimentally. In order to quantify these concepts, a value of the order of 0.1 may be adopted for the first low threshold, and a value of the order of 0.5 for the first high threshold.
The method according to the invention permits, if the forces of one of the axles do not correspond to the desired lateral forces, or if the effective yaw moment is greater than the desired yaw moment, or if the gain in the yaw velocity does not correspond to what is regarded as normal, the transmission of an action signal to the operating means in order to minimize the error signal without the need, either to establish such a signal or to measure the yaw velocity of the vehicle. It is understood of course that such a method is compatible with measuring the yaw velocity, particularly if it is desired to add redundancy terms to the calculations.
As can be seen, the invention provides a method for regulating a system for controlling the stability of a vehicle based on the forces acting at the center of each wheel of the vehicle. In fact, the actions of the driver, whether they involve steering, accelerating or braking, will become forces (variations in forces) transmitted by the tires to the ground. Depending on whether or not these variations of forces are compatible compared to the commands of the driver, it may be concluded whether or not the vehicle is stable. The resultant displacements are calculated on the basis of the forces acting on the ground. In this way it is possible to correct the trajectory of the vehicle much earlier and an ESP system gains in fineness of correction. Safety is better and the comfort of the driver and passengers is improved.
The estimation of stability criteria in real time, based on forces on the ground, enables the control of the stability of the trajectory of a vehicle to be improved, and the direct measurement of the force enables, for example, the saturation point of the pneumatic tire to be monitored accurately regardless of the grip on the road surface, by detecting the occurrence of non-linearity between the developed lateral force and the angle of sideslip of the tire in question.
The cause of loss of stability of the vehicle is mainly the fact that the tires are no longer able to correct the trajectory, given the movement of the vehicle. Irrespective of the lateral force developed by the tires, this lateral force will never be able to counteract the forces of inertia. This may be due to a poor grip (wet road, (black) ice, snow, sand, dead leaves), to the fact that the tire is used by the driver under improper conditions (flat tire or underinflated tire), or to the fact that the vehicle is directly placed in a situation of excessive drift or sideslip that exceeds the physical limits of one or more of the tires. In this case it may be said that one or more of the tires reaches its saturation point.
The suspension bearings may be equipped with instruments, as proposed in patent application JP60/205037, which enables the longitudinal and lateral forces developed by the tire to be determined easily by measurements made on the suspension bearings. Alternatively, the tire itself is equipped with sensors for recording the forces of the tire on the ground. Measures may be adopted as explained for example in patent DE 39 37 966 or as discussed in U.S. Pat. No. 5,864,056 or in U.S. Pat. No. 5,502,433.
On the basis of the forces measured by one or other of the above methods, and from equilibrium equations of a suspension device, the forces acting at the center of each wheel may accordingly easily be calculated. Thus, in real time 3 forces X, Y and Z are available, which in particular enables the Y signal to be processed for the reasons explained in the present document.